This invention relates generally to the production of chlorine dioxide. More particularly the present invention relates to the electrochemical process and the electrolytic cell structure used to manufacture chlorine-free chlorine dioxide from dilute alkali metal chlorite solutions. A conductive salt is added to the aqueous chlorite feed solution that is used in the cell to produce the product chlorine-free chlorine dioxide solution. Chlorine dioxide is commercially employed as a bleaching, fumigating, sanitizing or sterilizing agent.
The chlorine dioxide can be used to replace chlorine and hypochlorite products more traditionally used in bleaching, sanitizing or sterilizing applications with resultant benefits. Chlorine dioxide is a more powerful sterilizing agent and requires lower dose levels than chlorine, at both low and at high pH levels, although it is not particularly stable at high pH levels. Chlorine dioxide produces lower levels of chlorinated organic compounds than chlorine when sterilizing raw water. Additionally, chlorine dioxide is less corrosive to metals and many polymers than chlorine.
The electrochemical production of chlorine dioxide is old and well known. U.S. Pat. No. 2,163,793 to J. O. Logan, issued June 27, 1939, discloses a process which electrolyzes solutions of an alkali metal chlorite containing an alkali metal chloride as an additional electrolyte for improving the conductivity of the solution. The process preferably electrolyzes concentrated chlorite solutions to produce gaseous chlorine dioxide in the anode compartment of an electrolytic cell having a porous diaphragm between the anode and the cathode compartments.
A process for electrolyzing an aqueous solution containing a chlorite and a water soluble salt of an inorganic oxy-acid other than sulfuric acid is disclosed in British Patent No. 714,828, published Sept. 1, 1954, by Farbenfabriken Bayer. Suitable soluble salts include sodium nitrate, sodium nitrite, sodium phosphate, sodium chlorate, sodium perchlorate, sodium carbonate and sodium acetate.
A process for producing chlorine dioxide by the electrolysis of a chlorite in the presence of a water soluble metal sulfate is taught by Rempel in U.S. Pat. No. 2,717,237, issued Sept. 6, 1955.
Japanese Patent No. 1866, published Mar. 16, 1966, by S. Saito et al teaches the use of a cylindrical electrolytic cell for chlorite solutions having a porcelain separator between the anode and the cathode. Air is used to strip the chlorine dioxide from the anolyte solution.
Japanese Patent Publication No. 81-158883, published Dec. 7, 1981, by M. Murakami et al describes an electrolytic process for producing chlorine dioxide by admixing a chlorite solution with a catholyte solution for a diaphragm or membrane cell to maintain the pH within the range of from about 4 to about 7 and electrolyzing the mixture in the anode compartment. The electrolyzed solution, at a pH of 2 or less, is then fed to a stripping tank where air is introduced to recover the chloride dioxide.
U.S. Pat. No. 4,542,008 to Capuano et al, issued Sept. 17, 1985, teaches a process for electrolyzing aqueous chlorite solutions where the sodium chlorite concentration in the anolyte is controlled by means of a photometric cell to maintain a concentration of about 0.8 to about 5% by weight. Capuano et al further teaches the use of carbon, graphite or titanium or tantalum anodes, the latter two having an electrochemically active coating. The cell is divided by a permselective cation exchange membrane.
A disadvantage of all of the above electrolytic processes is the production of chlorine dioxide in the anode compartment of the cell so that the chlorine dioxide must be recovered from the anolyte by stripping with air or some other appropriate means. If this stripping step is not accomplished, the conversion of chlorite to chlorine dioxide in the electrolyte is typically less than 20% and the direct use of the anolyte would be economically infeasible. Operation of these electrolytic processes under conditions where higher conversion rates are attempted by applying more current and lower electrolyte feed rates results in the formation of chlorate and/or free chlorine. Since chlorine is an undesirable contaminant and since the formation of chlorate is irreversible, there is a need to develop a process by which chlorite can be converted to chlorine dioxide efficiently without a separation step.
The use of chlorine dioxide solutions poses a significant problem because the generation of chlorine-free chlorine dioxide is complex and requires a number of purification steps. These steps may include the aforementioned stripping and the reabsorbing of chlorine dioxide from a generating solution to a receiving solution. A stream of air is frequently used for this purpose. However, operation of such a process is hazardous if the chlorine dioxide concentrations in the air become high enough to initiate spontaneous decomposition.
U.S. Pat. No. 4,683,039 to Twardowski et al describes another method of accomplishing this purification step by use of a gas-permeable hydrophobic membrane. This method reduces the risk of chlorine dioxide decomposition that requires additional costly equipment.
An additional problem encountered in the prior chlorine dioxide production methods, especially using dilute sodium chlorite solutions, is the less than fully effective use of the electrode surface during the electrolytic production of the product chlorine-free chlorine dioxide. This is especially aggravated where high surface area electrodes are employed because of the expectation of high conversion performance of the chlorite to chlorine dioxide. In electrolytic cells using high surface area anodes, the highest electrolyte conductivity occurs near the bottom of the electrode structure and that conductivity decreases with height traveling up the structure towards the top of the electrode in the direction of upward flow of the electrolyte solution. This occurs because the conductive chlorite ion is oxidized to nonionized chlorine dioxide and the solution is transported through the separator towards the cathode. This phenomenon results in less efficient oxidation of the chlorite ions because of reduced charge transfer near the top of the high surface area electrode structure with a concurrent decrease in current density in this area. This, in turn, decreases the product yield obtained from the cell.
These and other problems are solved in the present invention by employing a conductive salt additive in the dilute alkali metal chlorite feed solution to an electrolytic cell in a continuous electrochemical process in the production of chlorine-free chlorine dioxide. The product chlorine-free chlorine dioxide is produced in concentrations of at least about 2 to about 10 grams per liter (gpL) and as much as about 14 gpL from dilute alkali metal chlorite solutions in a single step by use of a porous flow-through anode.